I was trying to create a Site2Site VPN from Azure to AWS using StrongSwan. All configurations were done and in Azure I need to create Local Network Gateway to Vnet Gateway. This Local network Gateway takes public IP of VPN server and Address space of that server subnet (private IP subnet x.x.x.x/y). Initially I created Local network Gateway with public IP and forgot to add address space. I tried tunneling and it got connected but not able to ping or reach other servers in other VPN.
Later I realized my mistake and tried adding Address space and it took few minutes to update the changes and later got failed. I searched many forums for this issue and no where it was mentioned about this Address space.
In my understanding this Address Space is mandatory for servers to communicate in a VPN. So, I created a new Local Network Gateway and added VPN Public IP along with the VPN subnet and everything started working.
Hope, this helps someone with same issue.